This invention relates to an apparatus for detecting the velocity of an object by using ultrasonic waves and in particular to an apparatus for measuring the blood flow speed in a living body in real time.
There are known various types of devices for measuring the flow speed of an object by use of the Doppler effect of acoustic waves. In particular, in an apparatus using the pulse Doppler method, by which the phase difference is detected, it is possible to measure the velocity of each of a plurality of parts over the whole measurement range in real time by sending transmitted pulses in the form of a burst and by measuring the phase difference of received signals for every interval of the transmitted pulses. An apparatus similar thereto is described e.g. in "Application of the phase detection principle in a transcutaneous velocity profile meter" by M. Brandestini, Proc. of the Second European Congress on Ultrasonics in Medicine, p. 144, 1975.
According to the pulse Doppler method described above, denoting the repetition period of the transmitted wave as T, the highest measurable Doppler frequency shift F.sub.d is equal to 1/2T. On the other hand, denoting the propagation velocity of an acoustic wave (sound velocity) as C, the measurable depth D is represented by TC/2. Consequently, the product of F.sub.d and D is constant and equal to C/4 and the measurable velocity or the measurable depth is therefore limited. For the purpose of increasing the highest measurable Doppler frequency shift F.sub.d, U.S. patent application No. 5,900 was filed Jan. 22, 1987, claiming priority based on Japanese patent application No. 61-10117 filed Jan. 22, 1986 and laid-open on July 25, 1987 (JP-A-62-169073). The corresponding DE patent application No. P3701786.1 was filed Jan. 22, 1987 and laid open on Aug. 6, 1987. In these patent applications it is proposed that transmitted pulse intervals are made unequal such as T-T.sub.s and T+T.sub.s and that the Doppler shift (.omega..sub.d) is obtained according to the following formula:
.omega..sub.d =(difference between phase differences)/2T.sub.s, where T.sub.s .noteq.0.
According to this method, it is possible to increase the highest measurable Doppler frequency shift by reducing T.sub.s. However, this method has problems such as degradation in the signal to noise ratio (S/N ratio), complication of the structure of moving target indicator filters (hereinbelow called MTI filters), etc. Concerning the former problem, denoting the unequal time interval ratio 2T.sub.s /T as .alpha., the noise component increases inversely proportionally to .alpha.. Concerning the latter, in the case where there are two sorts of unequal intervals, i.e. T-T.sub.s and T+T.sub.s, as described previously, so-called blind speeds, which are measured to be zero although the object is, in reality, moving, arise at n/(2T+T.sub.s) (n=1, 2, . . . ). In order to resolve this problem it is necessary to use more than 2 sorts of pulse intervals such as T-T.sub.s, T and T+T.sub.s, which complicates the structure of the MIT filters.
Reference may be made to "BLOOD FLOW IMAGING USING A DISCRETE-TIME FREQUENCY METER" by M. A. Brandestini et al., 1978 Ultrasonics Symposium Proceedings, IEEE Cat. #78 CH 1344-1SU.